1. Field of the Invention
The present invention is directed to a multiple stage turbine for use as a downhole motor on a drilling string, and more particularly, to a multiple stage turbine downhole motor which is driven by the drag or shear stress force alone or in combination with the dynamic or impulse force of the fluid flowing through the turbine.
2. Description of the Prior Art
Prior art downhole motors for use on drilling strings convert the kinetic energy of a mass of a fluid against the face surface of turbine blades into power for turning a drill string and thereby a drill bit attached to the bottom of the drill string. The turbines rely solely on the dynamic or impulse force. Prior art downhole motors of this type are generally required to be relatively long in order to have sufficient turbine blade surface area for generating enough power to turn the bit at the proper speed with sufficient torque. However, because the downhole motor itself is quite long, it is difficult for the drill string to move through curves and thus it is much more difficult to control the direction of drilling.
Another disadvantage of the dynamic force type downhole motors, is that maximum power and efficiency occur at rather high rotational speeds; higher than the range of operational speed for most mechanical drill bits, like tricone bits. The reason for this characteristic is that the functions of power and efficiency, in terms of the velocity of the flow is proportional to the square of the velocity. The function is a parabola in which the apex is approximately midway between zero and runaway or no load speed.
Still another disadvantage of prior art downhole turbine motors is that the turbine blades are internal with respect to the drilling shaft. In order to drive the turbine, fluid must flow through the internal structure of the drill string and can cause damage to the bearings, seals and other internal parts of the downhole motor.